The present invention relates to polymer blends comprising ethylene/propylene-derived copolymers and certain other propylene-derived polymers that exhibit pressure-sensitive adhesive properties. The pressure-sensitive adhesives are useful in preparing a wide variety of articles.
A wide variety of references describe blends of xcex1-olefin polymers (i.e., those polymers derived from at least one xcex1-olefin monomer) that are used in hot-melt adhesives and heat-sealing films. However, these references do not teach formulation of such adhesives so that they possess the balance of properties (e.g., shear, peel, and tack) requisite of PSAs. For example, see U.S. Pat. No. 3,492,372 (Flanagan); U.S. Pat. No. 3,798,118 (Jones); U.S. Pat. No. 3,900,694 (Jurrens); U.S. Pat. No. 4,178,272 (Meyer, Jr. et al.); U.S. Pat. No. 4,761,450 (Lakshmanan et al.); U.S. Pat. No. 4,857,594 (Lakshmanan et al.); U.S. Pat. No. 4,957,968 (Adur et al.); U.S. Pat. No. 5,397,843 (Lakshmanan et al.) ; and U.S. Pat. No. 5,834,562 (Silvestri et al.). Further examples include Japanese Patent Publication Nos. 60-120775; 55-069637; 48-066638; and 73-027739.
Pressure-sensitive adhesive (PSA) compositions are well known to those of ordinary skill in the art to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength. Materials that have been found to function well as PSAs are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. Obtaining the proper balance of properties is not a simple process.
The most commonly used polymers for preparing PSAs are natural rubber-, synthetic rubber- (e.g., styrene/butadiene copolymers (SBR) and styrene/isoprene/styrene (SIS) block copolymers), and various (meth)acrylate- (e.g., acrylate and methacrylate) based polymers. With the exception of several (meth)acrylate- and certain xcex1-olefin-based polymers, which are inherently tacky, these polymers are typically blended with appropriate tackifiers to render them pressure-sensitive.
PSA compositions comprising polymers derived from xcex1-olefin monomers are known. Often, such compositions include a single xcex1-olefin polymer, typically a copolymer, which is usually blended with a tackifier. Blending an xcex1-olefin-derived polymer with another polymer when preparing PSAs is known, however. Yet, the relative proportions of the polymers and the physical characteristics of the polymers in these blends is typically not specified.
For example, see European Patent Application No. 0 178 062 (Uniroyal, Inc.) teaches hot melt pressure-sensitive thermoplastic elastomeric adhesive compositions comprising blends of thermoplastic elastomeric polymers made from ethylene and propylene (e.g., those comprising crystalline polypropylene moieties and essentially amorphous elastomeric ethylene-propylene copolymer moieties having a melting point of at least 150xc2x0 C. as measured by differential thermal analysis), tackifiers, and other optional additives, such as plasticizers, fillers, and stabilizers. Amorphous polypropylene having a ring-and-ball softening point of 149-154xc2x0 C. is mentioned as an example of a suitable plasticizer. Plasticizers can comprise 5-50% by weight of the total adhesive.
Also see U.S. Pat. Nos. 3,954,697 and 4,072,812 (McConnell et al.), which teach single component, hot-melt, pressure-sensitive adhesives based on copolymers derived from propylene and 40-60 mole % (based on the copolymer) higher olefin monomers. The copolymers may be used alone or in mixture with other materials, including compatible tackifiers, amorphous polypropylene and amorphous block ethylene/propylene-derived copolymers.
U.S. Pat. No. 4,143,858 (Schmidt, III et al.) teaches amorphous polyolefin homopolymers and copolymers useful in hot-melt PSAs. The amorphous polyolefin polymers may also be blended with up to 20 weight % crystalline polyolefins. The adhesives therein may be used alone or in mixture with other materials such as amorphous polypropylene, amorphous block ethylene/propylene-derived copolymer, etc.
It is also noteworthy that many references describing blends of xcex1-olefin derived polymers with other polymers teach blending xcex1-olefin-derived polymers with other polymers that are rubber-based, containing conjugated dienes. For example, see U.S. Pat. No. 5,859,114 (Davis et al.), where a major amount of an ethylene-propylene-diene terpolymer is blended with a minor amount of at least one fully saturated adhesive-enhancing polymer miscible therewith and at least one tackifier. Examples of adhesive-enhancing polymers taught to be useful therein include: ethylene-vinyl acetate copolymers, ethylene-vinyl chloride copolymers, ethylene-octene copolymers, ethylene-butene copolymers, and propylene-butene-ethylene terpolymers. As described in U.S. Pat. No. 5,504,136 (Davis et al.), ethylene-propylene copolymers, such as those typically having an ethylene content of about 45-65% by weight, are also mentioned as having utility as the adhesive-enhancing polymer in a related invention.
PCT Publication No. WO 98/33,858 (American Tape Co.) describes PSAs comprising a natural or synthetic rubber and a thermoplastic polyolefin blend of ethylene/propylene rubber, hydrogenated polyisoprene, and polypropylene. Each of the constituents making up the thermoplastic polyolefin blend has a relatively low molecular weight (3,000-10,000). The PSA composition also includes a tackifier.
PCT Publication No. WO 97/23,577 (Minnesota Mining and Manufacturing Co.) describes blended PSAs that include at least two components. The first component is a PSA. For example, PSAs useful in the invention include tackified natural rubbers, synthetic rubbers, tackified styrene block copolymers, polyvinyl ethers, acrylics, poly-xcex1-olefins (described as being predominantly amorphous), and silicones. The second component is a thermoplastic material or elastomer. For example, thermoplastic materials useful in the invention include isotactic polypropylene and ethylene/propylene copolymers. It is also taught that useful thermoplastic materials are essentially immiscible in the PSA component at use temperatures. The Abstract describes the blends therein as having a substantially continuous domain (generally the PSA) and a substantially fibrinous to schistose domain (generally the thermoplastic material). Tackifiers may be added.
UK Pat. Application No. GB 2 041 949 (The Kendall Company) discloses PSAs comprising (1) a rubbery copolymer of ethylene, a C3-C14 monoolefin and, optionally, a non-conjugated diene, (2) a polymer of a crystalline ethylene or propylene, and (3) a tackifying agent.
Many compositions derived from diene monomers are relatively unstable over time, such as for example, when exposed to weathering or higher temperatures (e.g., such as when hot-melt processing the compositions). Furthermore, many such compositions are relatively non-polar and do not adhere adequately to both relatively high surface energy substrates and low surface energy substrates. xe2x80x9cLow surface energy substratesxe2x80x9d are those that have a surface energy of less than 45 mJ/m2, more typically less than 40 mJ/m2 or less than 35 mJ/m2. Included among such materials are polyethylene, polypropylene, acrylonitrile-butadiene-styrene, and polyamide.
Ways to effectively adhere to low surface energy materials is a challenge that those of ordinary skill in the art are attempting to overcome. Many times improvements in adherence to low surface energy substrates compromises adherence to higher surface energy substrates or compromises shear strength of the adhesive. As such, further adhesives for adequately adhering to low surface energy surfaces are desired. It is also desired that any such new adhesives will allow for broad formulation latitude and tailorability for particular applications.
It would also be desirable to formulate adhesives using lower cost materials. For example, (meth)acrylate monomers and rubber-based monomers are generally more expensive than xcex1-olefin monomers. The present invention addresses these motivating factors.
The present invention provides an improved pressure-sensitive adhesive composition which comprises a blend of at least one ethylene/propylene-derived copolymer and at least one propylene-derived polymer. Generally, such compositions are advantageous from a cost-standpoint, as opposed to many traditional pressure-sensitive adhesive compositions (e.g., (meth)acrylate and rubber-based adhesive). Furthermore, xcex1-olefin-derived polymers, which include ethylene/propylene-derived copolymers and propylene-derived polymers, are conducive to being recycled, particularly when used in conjunction with other xcex1-olefin-derived polymers (e.g., materials commonly used in packaging). It is beneficial to use materials in adhesive compositions that can be later recycled.
The blends of the present invention are useful in adhering to various substrates including relatively high surface energy substrates, such as glass and metals, as well as low surface energy substrates, such as polyethylene and polypropylene. They may be used in a wide variety of applications, such as in adhesive tapes and sheets and in the application of polymeric films to a wide variety of substrates. They may also be used in the preparation of blown microfiber webs.
Surprisingly, in preferred embodiments of the invention, peel adhesion to low surface energy substrates, such as polyethylene and polypropylene, is enhanced without causing detrimental effects in peel adhesion to high surface energy substrates. For example, certain embodiments of the invention provide substrates, which can be polypropylene, polyethylene, or glass, with the pressure-sensitive adhesive composition at least partially applied thereon. In these embodiments, the 180xc2x0 peel adhesion to the substrate can be as high as at least about 100 N/dm. Useful shear strengths are also realizable using the blends of this invention.
Pressure-sensitive-adhesive (PSA) blends of the present invention comprise at least one ethylene/propylene-derived copolymer and at least one propylene-derived polymer. Preferably, the PSA blend comprises a single domain system. Single domain systems, as well as other terms used throughout, are defined in turn below.
xe2x80x9cSingle Domainxe2x80x9d systems are those where, when analyzed using Dynamic Mechanical Analysis (DMA) in a parallel plate geometry at a temperature increment of 2xc2x0 C./minute, a frequency of 1 radian/second, and a maximum strain of 2%, only one tan delta peak (without a xe2x80x9cshoulderxe2x80x9d) representing a glass transition temperature is present. Typically, these systems involve polymers that form a miscible system at use temperature (e.g., room temperature). xe2x80x9cMiscible systemsxe2x80x9d are those systems comprising at least two materials forming a single domain system. It is advantageous to have single domain systems. For example, single domain systems often do not require additives (e.g., compatibilizers) to ensure storage stability of the system. Also, single domain systems facilitate reproducible compositions (i.e., the compositions have similar mechanical properties), even when the compositions are prepared using a range of different processing equipment.
xe2x80x9cPolymerxe2x80x9d refers to macromolecular materials having at least five repeating monomeric units, which may or may not be the same. The term xe2x80x9cpolymerxe2x80x9d, as used herein, encompasses homopolymers and copolymers. Copolymers of the invention refer to those polymers derived from at least two chemically different monomers. Included within the definition of copolymers are traditional copolymers derived from at least five monomers, which include only two chemically different types of monomers, as well as terpolymers, which include at least three chemically different types of monomers, etc.
In general, a polymer can include more than one type of steric structure throughout its chain length. For example, polymers can include crystalline, stereoregular isotactic and syndiotactic structures, as well as amorphous, atactic structures, or combinations thereof. The steric structure of a polymer can be determined using any suitable method. For example, carbon-13 Nuclear Magnetic Resonance can be used to determine the steric structure (i.e., tacticity) of a polymer.
xe2x80x9cStereoregularxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose molecular structure has a definite spatial arrangement, rather than the random and varying arrangement that characterizes an amorphous polymer. Stereoregular structures include isotactic and syndiotactic structures.
xe2x80x9cIsotacticxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose structure is such that groups of atoms that are not part of the backbone structure are located either all above, or all below, atoms in the backbone chain, when the latter are all in one plane.
xe2x80x9cSyndiotacticxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose structure is such that groups of atoms that are not part of the backbone structure are located in some symmetrical and recurring fashion above and below the atoms in the backbone chain, when the latter are all in one plane.
xe2x80x9cAtacticxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose structure is such that groups of atoms are arranged randomly above and below the backbone chain of atoms, when the latter are all in one plane.
The xe2x80x9cStereoregular Index (S.I.)xe2x80x9d of a polymer is defined as follows: In a perfectly atactic polymer, two homotactic triads, mm and rr, are present in equal amounts (25% each). As the polymer becomes increasingly stereoregular, the relative amounts of mm and rr change so that one increases to be greater than the other. S.I. is the ratio of the larger of mm or rr to the smaller of mm or rr and is always positive and greater than 1. S.I. expresses, in a numerical way, how the steric structure of a polymer shifts away from 1.0 for a random, atactic polymer to larger values characteristic of more stereoregular polymers.
xe2x80x9cNon-stereoregularxe2x80x9d polymers are generally mostly atactic or mostly semi-syndiotactic polymers, rather than mostly isotactic or mostly syndiotactic.
xe2x80x9cSemi-syndiotacticxe2x80x9d polymers are those having structures between mostly syndiotactic polymers and mostly atactic polymers.
In one embodiment, non-stereoregular polymers of the invention have an S.I. of 1 to about 10. In another embodiment, non-stereoregular polymers of the invention have an S.I. of 1 to about 7. In still a further embodiment, non-stereoregular polymers of the invention have an S.I. of 1.5 to about 7. In yet another embodiment, non-stereoregular polymers of the invention have an S.I. of 1 to about 1.1.
xe2x80x9cAmorphousxe2x80x9d polymers are those polymers that are hexane soluble at room temperature. Recognize that such materials may have a small degree of crystallinity, which is detectable, for example, using x-ray or thermal analysis. Amorphous polymers lack a well-defined melting point when measured by Differential Scanning Calorimetry (DSC). Particularly preferred are those amorphous polymers having mostly atactic structures.
Polymers that have less stereoregularity have been found to be preferred for processing and preparing PSAs of the invention, such as for example, by hot-melt processing. Furthermore, materials that are highly isotactic tend to be opaque, while those that are less stereoregular tend to be more transparent. The clarity (i.e., transparency) of materials with low stereoregularity makes them preferred for use in applications where clarity of the adhesive is important. For example, such applications include bonding of glass and transparent plastics.
One advantage of utilizing blends of the invention is the greater formulation latitude that they provide. That is, changes in a wide variety of physical properties of films comprising the blends can be effectuated, for example, by varying the ratio of individual polymers in the blends. Furthermore, cost effectiveness is another advantage of utilizing blends. For example, less expensive polymers can be blended with more expensive polymers. In that way, the less expensive polymers can act as an xe2x80x9cextenderxe2x80x9d for the more expensive polymers. Also, using blends can provide advantageous synergistic effects, wherein, for a certain application, the blend can perform substantially better than either polymer by itself for the same application.
PSA blends of the invention are particularly useful for adhering to both relatively high and low surface energy materials. PSA blends of the invention are capable of providing adequate or improved peel adhesion to such substrates. For example, certain embodiments of the invention provide substrates, which can be polypropylene, polyethylene, or glass, with the pressure-sensitive adhesive composition at least partially applied thereon. In these embodiments, the 180xc2x0 peel adhesion to the substrate can be as high as at least about 100 N/dm. Certain PSA formulations of the invention are also capable of providing compositions having useful shear strengths.
Ethylene/Propylene-Derived Copolymer
Any suitable ethylene/propylene-derived copolymer can be used in the present invention. Generally, however, the ethylene/propylene-derived copolymers of the invention are amorphous. As such, the compositions of the invention are able to have enhanced pressure-sensitive adhesive properties, often without the need for using substantial amounts of additives, such as plasticizers or liquid oils. The ethylene/propylene-derived copolymers themselves may or may not have pressure sensitive-adhesive properties. Furthermore, the use of amorphous ethylene/propylene-derived copolymers facilitates obtaining a single domain system. In contrast, when only crystalline xcex1-olefin polymers are used instead of at least one amorphous ethylene/propylene-derived copolymer, crystallization-induced phase separation may be encountered in the composition, impairing obtainment of a single domain system.
Another preferred aspect of the invention relates to the type of ethylene/propylene-derived copolymer used. While any suitable ethylene /propylene-derived copolymer, including block- and random-copolymers, can be used in accordance with the present invention, the use of at least one random-ethylene /propylene-derived copolymer is preferred. As with using amorphous ethylene/propylene-derived copolymers, the fact that the copolymer is random in a preferred embodiment also facilitates obtainment of a single domain system.
The ethylene/propylene-derived copolymer is derived from at least one ethylene monomer and at least one propylene monomer. While other monomers, including diene monomers, may be copolymerized with the ethylene and propylene monomers, when preparing the ethylene/propylene-derived copolymers, the ethylene/propylene-derived copolymer of the invention is derived from essentially no diene monomers. As discussed previously, many compositions derived from diene monomers are relatively unstable over time, such as for example, when exposed to weathering or higher temperatures (e.g., when hot-melt processing). Furthermore, many compositions derived from diene monomers are relatively non-polar and do not adhere adequately to both relatively high surface energy substrates and low surface energy substrates.
It is preferred that the ethylene/propylene-derived copolymer is derived from a major portion of propylene monomers. That is, the largest mole % of monomers from which the ethylene/propylene-derived copolymer is derived is that for propylene monomers. Similarly, it is preferred that the ethylene/propylene-derived copolymer contains less than about 35% by weight, more preferably less than 30% by weight, even more preferably less than about 25% by weight, and even more preferably less than about 15% by weight repeat monomeric units derived from ethylene monomers.
Any suitable amount of ethylene monomer may be used to prepare the ethylene/propylene-derived copolymer as long as the resulting copolymer is amorphous. Generally, however, the greater the proportion of ethylene monomer used, the more likely it is that the resulting copolymer will not be amorphous.
Particularly useful are the ethylene/propylene-derived copolymers with a glass transition temperature (Tg) of about xe2x88x9250xc2x0 C. to about 0 C., preferably greater than xe2x88x9240xc2x0 C. to about 0xc2x0 C., and more preferably about xe2x88x9230xc2x0 C. to about 0xc2x0 C. Generally, when the Tg of the ethylene/propylene-derived copolymer is lower than xe2x88x9250xc2x0 C., it is because a larger proportion of ethylene monomer was used in preparation of the copolymer. While some such copolymers may be useful for certain embodiments of the invention, as discussed above, these polymers may not be amorphous. Furthermore, it is preferred that the Tg of the ethylene/propylene-derived copolymer is greater than about xe2x88x9250xc2x0 C. in order to reduce the necessity for adding a tackifier, or at least a large amount of tackifier, to the composition in order to obtain PSA properties for room temperature applications. The Tg of a polymer is measurable using Differential Scanning Calorimetry using second heat measurements at 10xc2x0 C. per minute.
Examples of ethylene/propylene-derived copolymers useful in the present invention include polymers commercially available from Eastman Chemical; Kingsport, TN under the EASTOFLEX tradename and polymers commercially available from The International Group; Wayne, PA under the KTAC.tradename. Specific examples of suitable ethylene/propylene-derived copolymers from these companies are those with a Tg of about xe2x88x9233xc2x0 C. to about xe2x88x9223xc2x0 C., such as EASTOFLEX E1060, EASTOFLEX E1200, and KTAC6013.
Propylene-Derived Polymer
Any suitable polymer can be used for the propylene-derived polymer. The propylene-derived polymers themselves, may or may not have PSA properties. Generally, the propylene-derived polymer is non-stereoregular.
The propylene-derived polymer is derived from at least propylene monomer. While other types of monomers may be used in their preparation, preferably, the propylene-derived polymer is derived from at least 60 percent by weight, more preferably at least about 80 percent by weight, and most preferably essentially 100 percent by weight, propylene monomers. As such, it is also preferred that the propylene-derived polymer contains a saturated hydrocarbon backbone. Accordingly, preferably the propylene-derived polymer is derived from essentially no diene monomers. As discussed previously, many compositions derived from diene monomers are relatively unstable over time, such as for example, when exposed to weathering or higher temperatures (e.g., when hot-melt processing). Furthermore, many compositions derived from diene monomers are relatively non-polar and do not adhere adequately to both relatively high surface energy substrates and low surface energy substrates.
Propylene-derived polymers of the invention are of high enough molecular weight that they do not act as a tackifier or plasticizer. That is, the weight average molecular weight of the propylene-derived polymer is at least about 10,000 grams/mole. Preferably, the weight average molecular weight of the propylene-derived polymer is at least about 30,000 grams/mole, even more preferably at least about 50,000 grams/mole, and even more preferably at least about 70,000 grams/mole. Particularly useful are polymers with a weight average molecular weight of about 70,000-1,000,000 grams/mole, preferably about 70,000-200,000 grams/mole.
The preferred Tg of these polymers is about xe2x88x9215xc2x0 C. to about 10xc2x0 C., more preferably about xe2x88x9210xc2x0 C. to about 5xc2x0 C. The use of at least one propylene-derived polymer having such a preferred Tg facilitates formation of a composition having PSA properties. Again, the Tg of a polymer is measurable using Differential Scanning Calorimetry using second heat measurements at 10xc2x0 C. per minute.
The preferred melt viscosity of the propylene-derived polymer is greater than 500 Poise, more preferably greater than about 750 Poise, when measured at 190xc2x0 C. according to the Viscosity Test method in the Examples section, infra. In a further embodiment, the melt viscosity of the propylene-derived polymer is greater than about 2,500 Poise when measured at 190xc2x0 C. according to the Viscosity Test method. In still a further embodiment, the melt viscosity of the propylene-derived polymer is greater than about 10,000 Poise when measured at 190xc2x0 C. according to the Viscosity Test method. Generally, the higher the melt viscosity of the propylene-derived polymer, the more likely it is that the resulting composition will have a higher shear strength in conjunction with improved peel adhesion properties. This is particularly beneficial when preparing PSA compositions of the invention for high performance applications.
Any suitable propylene-derived polymer can be used in blends of the invention. When higher molecular weight propylene-derived polymers are preferred, those polymers prepared using a metallocene catalyst, such as in PCT Publication No. WO 99/20,664, are particularly useful. Typically, polymers prepared using a metallocene catalyst (i.e., metallocene-generated polymers) have a weight average molecular weight of greater than about 70,000 grams/mole, which is typically higher than the molecular weight of many commercially available non-stereoregular propylene-derived polymers. A similar comparison applies when comparing melt viscosities of the polymers. Propylene-derived polymers prepared using metallocene catalysts may be preferred when PSA compositions having higher shear strength are desired in conjunction with improved peel adhesion properties. The higher molecular weight of the propylene-derived polymers prepared using a metallocene catalyst also enables them to be more usefully crosslinked, as compared to those propylene-derived polymers having lower molecular weights. This may be the case, when for example, the PSAs are to be used in a high performance application.
As stated previously, however, one advantage of the present invention is that the blends are tailorable for a wide variety of applications. Higher molecular weight polymers may not always be preferred depending on the application. For example, lower molecular weight polymers may be preferred when using the PSA composition to form a melt-blown fiber. PSA blends of the invention may be advantageously used to prepare blown microfiber webs, for example. Addition of a lower molecular weight polymer to a conventional polymer composition in accordance with the invention tends to lower the melt viscosity of the polymer composition at a given processing temperature. Therefore, the use of polymer blends of the invention may facilitate melt blowing fibers from PSA compositions at lower temperatures than those used to melt-blow fibers from conventional PSA compositions. Also, the use of polymer blends of the invention may facilitate a higher throughput of melt-blown fibers at a given processing temperature.
To facilitate forming a single domain PSA system, it is preferred that the propylene-derived polymer be a non-stereoregular polymer. That is another reason why metallocene-catalyzed propylene-derived polymers, such as those described in PCT Publication No. WO 99/20,664, are useful due to their generally lower stereoregularity. Such polymers are generally either amorphous or semi-syndiotactic. However, in this preferred embodiment, any suitable non-stereoregular polymer can be used.
According to one aspect of the invention, the stereoregularity index (S.I.) of the propylene-derived polymer is about 1.0 to about 5.0. Preferably, when the propylene-derived polymer is amorphous, its S.I. is about 1.0 to about 1.05. Preferably, when the propylene-derived polymer is semi-syndiotactic, its stereoregularity index (S.I.) is about 1.1 to about 4.0.
Optional Tackifier
Tackifiers of the invention have a weight average molecular weight of less than about 10,000 grams/mole and may be a in a solid or liquid state. The compositions of the invention may include a tackifier, where necessary to impart the desired PSA properties. Those of ordinary skill in the art recognize that a wide variety of tackifier are suitable for this purpose. Although a tackifier can be used as understood by one of ordinary skill in the art, generally, if present, the compositions include less than about 60% by weight tackifiers.
Preparation of Blends
PSA compositions of the invention include at least one ethylene/propylene-derived copolymer and at least one propylene-derived polymer. Other additives (e.g., antioxidants and ultraviolet stabilizers) may also be added to the PSA compositions, depending on the desired application and as well known to one of ordinary skill in the art.
Each of the ethylene/propylene-derived copolymer and propylene-derived polymer components of the blend is preferably present in an amount of about 5 weight % to about 95 weight % based on total weight of the blend. More preferably, each of the components is present in an amount of at least about 10 weight % based on total weight of the blend. Typically, however, the ethylene/propylene-derived copolymer component is present in a major portion and the propylene-derived polymer component is present in a minor portion based on total weight of the two components. This ratio of components contributes to cost-effectiveness and easier hot-melt processability of the composition. Furthermore, this ratio of components contributes to obtainment of compositions having adequate adhesion to both relatively high surface energy substrates and low surface energy substrates. It has also been found that this ratio facilitates formation of compositions having PSA properties by helping to lower the overall Tg of the composition.
Blending of the polymers is done by any method that results in a substantially homogeneous distribution of the polymers. The polymers can be blended using several methods. In particular, the polymers can be blended by melt blending, solvent blending, or any suitable physical means.
For example, the polymers can be melt blended by a method as described by Guerin et al. in U.S. Patent No. 4,152,189. That is, all solvent (if used) is removed from each polymer by heating to a temperature of about 150xc2x0 C. to about 175xc2x0 C. at a pressure of about 5 Torr to about 10 Torr. Then, the polymers are weighed into a vessel in the desired proportions. The blend is then formed by heating the contents of the vessel to about 175xc2x0 C., while stirring.
Although melt blending is preferred, the PSA blends of the present invention can also be processed using solvent blending. In that case, the polymers in the blend should be substantially soluble in the solvents used.
Physical blending devices that provide dispersive mixing, distributive mixing, or a combination of dispersive and distributive mixing are useful in preparing homogenous blends. Both batch and continuous methods of physical blending can be used. Examples of batch methods include those methods using BRABENDER (e.g., a BRABENDER PREP CENTER, available from C. W. Brabender Instruments, Inc.; South Hackensack, N.J.) or BANBURY internal mixing and roll milling (available from FARREL COMPANY; Ansonia, Conn.) equipment. Examples of continuous methods include single screw extruding, twin screw extruding, disk extruding, reciprocating single screw extruding, and pin barrel single screw extruding.
Applications
The PSA compositions of the present invention can be readily applied to a substrate. For example, the PSA composition can be applied to sheeting products (e.g., decorative, reflective, and graphical), labelstock, and tape backings. The substrate can be any suitable type of material depending on the desired application. Typically, the substrate comprises a nonwoven, paper, polymeric film (e.g., polypropylene (e.g., biaxially oriented polypropylene (BOPP)), polyethylene, polyurea, or polyester (e.g., polyethylene terephthalate (PET)), or release liner (e.g., siliconized liner).
PSA compositions according to the present invention can be utilized to form tape, for example. The PSA is applied to at least one side of the backing. The PSA may then be crosslinked to further improve the shear strength of the PSA. Any suitable crosslinking method (e.g., exposure to radiation, such as ultraviolet or electron beam) or crosslinker additive (e.g., phenolic and silane curatives) may be utilized.
When double-sided tapes are formed, the PSA is applied to at least a portion of both sides of the backing. Alternatively, a release material (e.g., low adhesion backsize) can be applied to the opposite side of the backing, if desired. Advantageously, the PSA and/or release material, for example, can be coextruded with the film backing for ease of processing.
The PSA can be applied to a substrate using methods well known to one of ordinary skill in the art. For example, the PSA can be applied using melt extrusion techniques. The PSA composition can be applied by either continuous or batch processes. An example of a batch process is the placement of a portion of the PSA composition between a substrate to which the PSA is to be adhered and a surface capable of releasing the PSA to form a composite structure. The composite structure can then be compressed at a sufficient temperature and pressure to form a PSA layer of a desired thickness after cooling. Alternatively, the PSA composition can be compressed between two release surfaces and cooled to form, for example, a transfer tape.
Continuous forming methods include drawing the PSA composition out of a heated film die and subsequently contacting the drawn composition to a moving plastic web or other suitable substrate. A related continuous forming method involves extruding the PSA composition and a coextruded release material and/or backing from a film die and cooling the layered product to form an adhesive tape. Other continuous forming methods involve directly contacting the PSA composition to a rapidly moving plastic web or other suitable preformed substrate. Using this method, the PSA composition is applied to the moving preformed web using a die having flexible die lips, such as a conventional film or sheeting die. After forming by any of these continuous methods, the films or layers can be solidified by quenching using both direct methods (e.g., chill rolls or water baths) and indirect methods (e.g., air or gas impingement). Blown microfibers can also be prepared using another continuous forming method. Examples of this process can be found, for example, in PCT Publication No. WO 99/28,539.
Although coating out of solvent is not preferred, the PSA compositions can be coated using a solvent-based method. For example, the PSA composition can be coated by such methods as knife coating, roll coating, gravure coating, rod coating, curtain coating, and air knife coating. The coated solvent-based PSA composition is then dried to remove the solvent. Preferably, the applied solvent-based PSA composition is subjected to elevated temperatures, such as those supplied by an oven, to expedite drying.
The PSA compositions, coatings, and tapes therefrom are exemplified in the following examples. These examples are merely for illustrative purposes and are not meant to be limiting to the scope of the appended claims. All parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight unless indicated otherwise.